Method of alloying aluminum and copper



United States Patent O METHOD OF ALLOYING ALUMINUM AND COPPER Dean K. Hanink, Indianapolis, Ind., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Filed Sept. 10, 1957, Ser. No. 683,008

2 Claims. (Cl. 204-38) This invention relates to an electrolytic method of alloying aluminum with copper employing a molten salt electrolyte.

In the commercial process for chromium plating, it is customary to employ only a very thin coating of chromium over the base metal. Such coatings are so thin that they are not only highly transparent but the underlying base metal can corrode quite readily. It has therefore been the practice in the art to employ a corrosionresistant intermediate layer to serve as a barrier to corrosion as well as lend to the silvery appearance of the decorative plate. In the chromium plating of ferrous articles, for example, a copper layer is applied immediately adjacent the surface of the base metal and a nickel coating is applied over the copper. The chromium is then deposited over the nickel to obtain a product having a relatively high resistance to corrosion and a decorative silvery appearance with only a very thin outer coating of chromium.

It is desirable in many commercial applications of chromium plating to reduce or eliminate the relatively costly intermediate underlying layer of nickel which is presently used in the commercial chromium plating process. One method by which an intermediate layer of nickel can be reduced or eliminated by the alloying of the copper layer with an aluminum difiusion coating. Heretofore, aluminum diffusion coatings have been applied by a spraying or hot dipping process followed by a heat treatment dii'rusing the aluminum into the copper. These methods generally are unsuitable since they are difiicult to control, usually resulting in rough surfaces or complete dissolving of the copper plate. Particularly, the reactive tendencies between copper and aluminum produce violent exothermic generations of heat in localized areas. The high temperatures created at these areas melt the aluminum-copper alloy, causing a surface glittering thereof.

Accordingly, an object of this invention is to provide a process for forming a thin, corrosion-resistant, copperaluminum alloy coating on a base metal article by means of an electroplating process which eliminates the aforementioned disadvantages. Another object of the invention is to provide a method of forming a thin, corrosionresistant, copper-aluminum alloy coating on a base metal article suitable for deposition of a decorative chromium plate.

These and other objects are attained in accordance with this invention by a process in which aluminum is electroplated from a molten salt electrolyte onto a coppercoated base metal.

Other objects, advantages and features of the present invention will become more apparent from the following description of a preferred embodiment thereof.

Metal articles of any shape can be coated with a thin layer of an aluminum-copper alloy by the method of this invention. The process can be a continuous one if desired, especially if the metal being treated is in the form of sheet, wire or tube stock.

In the coating of steel or other ferrous metal articles, for example, the metal article is preferably first degreased in any suitable manner as by means of an alkali cleaner or by the use of suitable solvents. After degreasing the metal, if it is severely corroded or scaled it is preferably pickled in an aqueous solution of hydrochloric acid in known manner. Subsequent to the pickling operation, the metal may be immersed in a flux such as one composed, by weight, of approximately 32 parts of zinc chloride, 8 parts of ammonium chloride and 60 parts of water. Such a flux is a typical example of a zinc chloride type flux which can be used.

The surface of the metal after fluxing is coated with a thin layer of copper in any known and accepted manner of deposition. A thin layer of about 0.0005 to 0.003 inch has proved to be satisfactory. The method of applying the thin copper coating is not considered to be particularly critical except in that the method should deposit a smooth, continuous, even coating of copper for best chromium plating results. A process which has been found to be particularly suitable for electroplating copper onto a steel involves the use of an aqueous copper electrolyte solution of the cyanide type. Electric current is passed through the electrolyte solution to the base member, immersed therein as the cathode, for a duration of time of at least approximately 50 seconds. The direction of current flow is then reversed, having the base member as the anode for a time interval less than the time interval that the base member Was cathodic. This cycle is repeated until the desired amount of copper has been electrodeposited onto the base member. The following is an example of a suitable aqueous copper plating bath of the cyanide type:

Ounces per gallon Copper 5.4 Free sodium cyanide 1.32 Potassium hydroxide 5.12 Potassium carbonate 10.8

In accordance with a preferred embodiment of this invention, after coating the base metal with a thin layer of copper employing a process such as that just described, the article is immersed in a molten salt electrolyte bath. The electrolyte salt is maintained at a temperature somewhat above the melting point of aluminum, preferably at about 1300 F. The copper-coated article is immersed in the molten salt bath and should be allowed to remain in contact therewith for a sufficient interval of time to heat the copper-coated metal approximately to or above the melting point of aluminum. When the copper-coated a specific salt bath that we have found to be highly I satisfactory, the percentages being by weight:

Percent Aluminum fluoride 10 Cryolite 10 Sodium chloride 40 Potassium chloride 40 The exact composition of the salt bath is not particularly critical and the proportions of aluminum fluoride,

from 25% to 45% and the potassium chloridefr om 37%.

to. 57%. The bathcomposition, usually preferred is one thatwill become, moltenwhen heated, to. 1200 F. or. somewhat lower.v While in the foregoing examples the double.lsalt,Na AlF (cryolite) isgiven, it should be understood that an equivalent amount of this component may be supplied. in the formof the single salts, sodium fluoride. and aluminumfiuoride. We have found, however, .thatit is essential to provide an excess of aluminum fluoride over, thatofthe cryolite ratio in .order to obtain the desiredsresults.

During, operation .the temperature of the. fused salt bath is maintained at approximately. 1250 F. to 1600 F. Below 1250 F. the molten salt electrolyte becomes less active with respect'to fiuxing ability, while above 1600 F. it becomes, highly. volatile and excessive loss due to vaporization may occur. In general, a temperature of about. 1300 -1 to 1400 F. is preferred, and in most instances, an operating temperature of. approximately 1300. F. has proven to be satisfactory.

Electrolysis. of the molten or fused salt electrolyte bath with the copper-coated metal article immersed therein under, a negative potential deposits aluminum onto the copper coating. As .the aluminum is deposited it is diffused into the surface of the heated metal. The rate.

of deposition of, the aluminum should be so regulated that'it is generally equal to the amount diffusing into the' surface of the copper. In this manner, no substantially separate overlay of aluminum over the copper is formed. The electrolysis is continued until the desired quantity of aluminum has been-deposited. After electrolyzing the molten bath for a sufficient time to obtain a suitablecopper-aluminum alloy coating, the flow of current therethrough is terminated. The aluminumcopper alloy coated metal article can be left in the molten salt bath for a short interval of time after termi nation of electrolysis in order to further difiuse the aluminum into the copper coating. Should the rate of diffusion equal or be greater than the rate of electrodeposition, the additional diffusing ofthe aluminum after electrodeposition is terminated would be unnecssary. However, in most instances, it is desirable to include this latter step to insure thorough diffusion of thealuminum into the'copper. Thereafter, thealuminum-copper alloy coated metal is removed .fromthe salt bath and cooled. Although other methods of cooling maybe suitable, satisfactory results are obtained when the aluminum-copper alloy coated article is cooled in air to generally about 600 F. and subsequently quenched in water. The cooled aluminum-copper alloy coated metal can then be washed in orderto remove the salts-from the molten salt bath which may adhere to the surface of the alloy coating. The excess electrolyte salt may be removed by washing, for example, or by other means, such as passingthe coated metal through rollers to remove the flux and to provide a desirable finish on the coated product. The alloy coated metal can then be electroplated with chromium in. any suntable manner, such as those known in the art.

The steps of degreasing, pickling, immersing in the zincchloride type of flux, and the method of copper.

plating are not essential to the processunlike the step of heating. priorv to the electrolysis operation. Heating of. the. copper-coated article in the described molten salt bath will normally provide a clean surface on the article Without further treatment. The metal can be preheated if desired before immersing in the fused salt'bath. In most instances, it preferred to heat the article under'such conditions that the surface thereof is not oxidized. Accordingly, heating in a. non-oxidizing or." reducing atmospheie furnace, such... as. .one using hydrogem. Dry:

colene, etc., can be employed. The preheating temperature is preferably generally similar to the temperature of the fused salt bath.

When the, metal to be coated is preheated to the temperature of the fused salt bath in a reducing atmosphere and is free of oxides andother foreign matter, the time of immersion in the salt bath prior to electrolysis may be as little as one or. two seconds. For parts. which have complicated shapes andcomplex recesses therein, a longer time may be required.to insure that the. fus ed salt has thoroughly contacted all-thesurfaces of the, metal article. When the metal article has extensive oxidetormation on the surface thereof, longer times will be required in order to cleansethe: surface and prepare it for the deposition of the aluminum Ifa separate preheating step is not employed prior to electrolysis, the metal article must be immersed in the molten flux electrolyte a sufiicient interval of time to bring themetal to the bath temperature. The duration of such; heating, of

course, will depend upon the dimensions of the metal and on the size and thermal efliciency of .the salt bath, I

The time of electrolysis of the metal in the fusedsalt bath may vary from as littleas a few seconds -up to a.

few minutes, depending upon thetdegree of'complexity of the recesses and the; dimensions of the' part, the current density used, etc. A relatively short period of electrodeposition of the aluminum on the metal surface is desirable in order to preventexcessive .forrnationof a high aluminum outer phase in the alloy layer in which the aluminum has not sufliciently been diffusedinto the surface of thebase metal.

The current density of theele'ctrolysis is not.particul'arly critical and canrange, for example, from approx-imately 0.1 ampere to 30 amperes per square inch. A complex part will have many high currentdensity areas inducing uneven plating and diffusion. Thus, lower average. current densities must be employed forsuch parts. Additionally, excessive potassium and sodium plate out on. the article, interfering with. the deposition of the aluminum whenan excessively high current density is employed. The rate which the aluminum difiuses into the base metal at the particular temperature used and the complexity of the .partwill primarily determine the preferred rate of electrodeposition.

Although the present invention has been illustrated in connection with certain specific examples thereof, it is not intended to be limited thereby except as defined by.

coating on the surface-of a'metal article which comprises the steps-of applying a copper coating having a thickness of about 0.0005 inchto 0.003 inch-to a surface of a metal article, applying to said copper coating. a fusedsalta bath comprising, by weight, approximately 0.5% to'12%- aluminum fluoride,..8% to 20%; cryolite, 25 to 45% potassium chloride and 37% to'5 7% sodium chloride, said fused salt bath being. at .a temperatureof approximately 1250. F. to 1600 1 inducing. a negative potential on said article while it is in contact .with-gsaid-bath: so as to electrodeposit aluminumonto' said copper. coating. the. amountof aluminumso depositedanot exceeding that which will alloy completely with the copper, .diffusing-.

said aluminum into said copper to completely alloy, it therewith,.coolingv said. article and removing any residual salts from. the surface thereof,

2. The. method of forming a copper-aluminum alloy coating on the surface of a metal article :which-comprises;

the steps of applyinga coppencoating having a thickness ofabout 0.0005 inch to 0.003 inchto a surface. of, a

metal article; heating. said. copper. coated article-to. a

temperature ofapproximately; 1250 F. to 1600 i. by

immersingit .inLa fused. salt ,bathrcomprising, ,by weight, .1 approximate y. 0.5% a min m .fl Q id 20% cryolite, 25% to 45% potassium chloride and 37% to completely alloy it therewith, cooling said article and to 57% sodium chloride, said fused salt bath being at a removing any residual salts from the surface thereof. temperature of approximately 1250" F. to 1600 F., in-

ducing a negative potential on said heated article while it References Cited In the file of this patent is immersed in said bath so as to electrodeposit aluminum 5 UNITED STATES PATENTS onto said copper coating, the amount of aluminum so 1,004,673 Monnot Oct. 3, 1911 deposited not exceeding that which will completely alloy 1,927,772 Chittum Sept. 19, 1933 with the copper, difiusing said aluminum into said copper 2,709,154 Hansgirg May, 24, 1955 UNITED STATES PATENT @FFICE CERTIFICA'HQN OF QECTION Patent No. 2 97O O9l January 311 1961 Dean Hanink It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 39, after "eliminated insert is column 3, line 62 for "suntable read suitable Signed and sealed this 13th day of June 1961.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer DAVID L. LADD Commissioner of Patents 

1. THE METHOD OF FORMING A COPPER-ALUMINUM ALLOY COATING ON THE SURFACE OF A METAL ARTICLE WHICH COMPRISES THE STEPS OF APPLYING A COPPER COATING HAVING A THICKNESS OF ANOUT 0.0005 INCH TO 0.003 INCH TO A SURFACE OF A METAL ARTICLE, APPLYING TO SAID COPPER COATING A FUSED SALT BATH COMPRISISING, BY WEIGHT, APPROXIMATELY 0.5% TO 12% ALUMINUM FLOURIDE, 8% TO 20% CRYOLITE, 25% TO 45% POTASSIUM CHLORIDE AND 37% TO 57% SODIUM CHLORIDE, SAID FUSED SALT BATH BEING AT A TEMPERATURE OF APPXOMATELY 1250*F. TO 1600*F., INDUCING A NEGATIVE POTENTIAL ON SAID ARTICLE WHILE IT IS IN CONTACT WITH SAID BATH SO AS TO ELECTRODEPOSIT ALUMINUM ONTO SAID COPPER COATING, THE AMOUNT OF ALUMINUM SO DEPOSITED NOT EXCEEDING THAT WHICH WILL ALLOY COMPLETELY WITH THE COPPER, DIFFUSING SAID ALUMINUM INTO SAID COPPER TO COMPLETELY ALLOY IT THEREWITH, COOLING SAID ARTICLE AND REMOVING ANY RESIDUAL SALTS FROM THE SURFACE THEREOF. 